When tabbing out once after you selected a spell, it gets unselected.

[mild bloom]

Happens when Fullscreen is off.

[next stratus]

Im not confirming/denying if this is intentional or not; it could be because tab is an alternate way to move your cursor and select/deselect things in game. So its possible the tab portion of alt tabbing gets read as tabbing to click somewhere on screen which would also deselect spells